1. Field of the Invention
The present invention relates generally to an exhaust emission control system of a hybrid car, and more particularly to an exhaust emission control system relative to an internal combustion engine of the hybrid car provided with two types of power sources such as an internal combustion engine and an electric motor and traveled by optimally combining driving forces generated by these two types of power sources corresponding to a condition.
2. Related Background Art
A hybrid car also adopts an internal combustion engine, such as a gasoline direct-injection lean burn engine and a diesel engine etc, exhibiting a high thermal efficiency and producing small quantities of hydrocarbon and carbon monoxide with an excessive oxygen in an atmosphere in an exhaust system.
For example, an occlusion reduction type lean NOx catalyst may be one of catalysts used in the exhaust system as an element for purifying an exhaust gas of the internal combustion engine described above. This occlusion reduction type lean NOx catalyst is categorized as an intermittent processing type lean NOx catalyst for temporarily occluding nitrogen oxide in a lean atmosphere; and, after being thereafter supplied with a proper amount of reducing agent, thereby promptly performing reduction-purging of the occluded nitrogen oxide. Note that there is a selective reduction type lean NOx catalyst other than the occlusion reduction type lean NOx catalyst. This catalyst is a consecutive processing type lean NOx catalyst which consecutively effecting selective reduction purging of the nitrogen oxide, by supplying a reduction agent. Further, hydrocarbon may be exemplified as a reducing agent as shown in, e.g., Japanese Patent Application Laid-Open Publication No. 6-117225. Moreover, carbon monoxide other than the hydrocarbon is known as a reducing agent.
When the hydrocarbon is used as the reducing agent, some proportion of the hydrocarbon is partially oxidated to produce an activated species. Then, this activated species reacts to the nitrogen oxide and reduces it, thereby producing nitrogen, hydrogen, oxygen and carbon dioxide which are harmless to a human body.
Further, the hydrocarbon is a main component constituting gasoline and a light oil serving as a fuel of the internal combustion engine. Hence, the supply of the hydrocarbon as the reducing agent to the lean NOx catalyst implies nothing but to supply the internal combustion engine with an extra fuel, apart from supplying the gasoline, the light oil and other internal combustion engine fuels for driving the engine as an essential use thereof. Note that the gasoline, the light oil and other internal combustion engine fuels are referred to as "fuels for combustion", as far as they are not particularly specified.
Then, according to the technology disclosed in the above Publication, the above extra supply is, separately from the supply through an injection at an expansion stroke by an engine fuel injection device such as an injector etc as normally done in the internal combustion engine, carried out at the expansion and exhaust strokes by the injection of the same engine fuel injection device. Then, of these injections, the former injection for driving the engine is called a main injection, and the other latter injection is called a sub-injection.
As described above, according to the prior art, both of the main injection and the sub-injection are performed by use of the same engine fuel injection device, and hence a greater number of injections are carried out by the engine fuel injection device than in the case of effecting only the main injection. Accordingly, a load on the engine fuel injection device increases, which might cause a fault in the engine fuel injection device.
Further, the hybrid car is provided with two types of power sources such as the internal combustion engine and the electric motor as explained above, and takes such a system as to travel by an optimum combination of the driving forces given from the two types of power sources corresponding to a condition. Therefore, the power source during the traveling of the vehicle may take three modes in which only the electric motor works, only the internal combustion engine works, and both of these two power sources work. Note that the driving of the internal combustion engine implies that the engine itself works by itself through four operating strokes; intake stroke, compression stroke, expansion (explosion) stroke, and exhaust stroke.
Generally, effective functioning of the catalyst requires activation of the catalyst, and, for attaining this function, the catalyst must be warmed up to a temperature enough to effectively make the catalyst function, i.e., up to a so-called activation temperature. Then, in a vehicle mounted with a conventional internal combustion engine, the temperature of catalyst is raised by utilizing the heat of exhaust gas emitted from the internal combustion engine. However, an exhaust gas is not produced during a halt of the internal combustion engine and, therefore the exhaust gas does not flow to the catalyst. Hence, heretofore, the catalyst becomes able to function effectively only after starting of operation of the internal combustion engine, and the catalyst could not have been warmed up to an activated temperature while the internal combustion engine was in a halt state, before starting the operation of the internal combustion engine. Thus, it can be assumed that the catalyst is not in the activation temperature immediately after the start of operation of the internal combustion engine.
This can be said for an internal combustion engine mounted on a hybrid car, that is, it is highly possible that the catalyst is not in the activated temperature immediately after starting of the operation of the engine. In this case, therefore, it may be possible that the catalyst cannot function effectively as exhaust gas purifying means for a while until it is activated. Further, if the hybrid car adopts the internal combustion engine, such as a direct injection engine and a diesel engine, exhibiting a high thermal efficiency and producing small quantities of hydrocarbon and carbon monoxide with an excessive oxygen in an atmosphere in an exhaust system, and when, for instance, the internal combustion engine is in an operation state such as a light load region, an exhaust gas temperature at that time is low with the result that heat enough to activate the catalyst is not transferred to the exhaust system, and on the contrary it might happen that the catalyst is cooled by a low-heat exhaust gas. Further, if there are the small quantities of hydrocarbon and carbon monoxide and the excessive oxygen in the atmosphere in the exhaust system, and further if the exhaust gas temperature is low, it is difficult to recover from S-poisoning of the catalyst.
Such being the case, if the atmosphere in the exhaust system is enriched by applying an external load to the internal combustion engine in order to increase the exhaust gas temperature for the purpose of purifying the exhaust gas and recovering the S-poisoning of the catalyst by activating the catalyst, this in turn becomes a factor for deteriorating a fuel consumption, and besides particulate matters might be produced.